Progressing cavity pumps have been used in water wells for many years. More recently, such pumps have been found to be well suited for the pumping of viscous or thick fluids such as crude oil laden with sand. Progressing cavity pumps include a stator which is attached to a production tubing and a rotor which is attached to the bottom end of a pump drive string and is made of metallic material, usually high strength steel.
Progressing cavity motors are used to provide rotary power sections for use in horizontal and directional drilling. Progressing cavity motors include a stator which is connected with a drillpipe and a rotor which is attached to a drill bit. Drilling fluid is forced down the drillpipe causing rotation of the rotor and operation of the motor to rotate the drill bit.
The rotor is usually electro-plated with chrome to resist abrasion. However, the corrosive and abrasive properties of the fluids produced in oil wells or utilized for drilling fluid frequently cause increased wear and premature failure of the rotor. Since it is important for efficient operation of the pump/motor that a high pressure differential be maintained across the rotor, only small variations in the rotor's dimensions are tolerable. This means that excessively worn rotors must be replaced immediately. However, replacement of the rotor requires pulling the whole pump/motor drive string from the well which is costly, especially in the deep oil well applications which are common for progressing cavity pumps/motors. Consequently, rotors with increased wear resistance and, thus, a longer service life are desired to decrease well drilling and operating costs.
Various hardfacing methods have been used in the past to increase the wear resistance of metal surfaces.
A number of progressing cavity pump/motor manufacturers chrome electroplate the rotors to increase wear resistance. Chrome electroplating does provide increased wear resistance, but is susceptible to corrosion in the harsh environment of downhole production and drilling.
Another way of increasing wear resistance is to deposit a coating or layer of material onto the rotor by thermal spraying. Conventional flame spraying uses a relatively low flame temperature and particle velocity (such as less than about 40 m/s), and results in coatings with high porosity and permeability as well as low bond strength. Nevertheless, it allows the spraying of a layer with much smaller thickness variations, overcoming the problem of uncontrollable thickness variations experienced with other thermal spraying techniques.
In general, conventional flame spraying techniques result in coatings with high porosity and permeability as well as low bond strength, although they do allow the spraying of a layer of sufficiently consistent thickness. Thickness variations on the other hand are a major problem with other coating techniques, such as high velocity oxygen fuel (HVOF) or detonation gun (D-gun) coating. Furthermore, those coating techniques cannot always be used to produce a sufficiently thick coating. In order to prevent failure of the coating during use, the thickness of the coating must be equal to at least 50% of the diameter of any particles to which the coating is exposed during use. Moreover, sufficiently thick coatings, even if achievable are subject to pitting and spelling during use, due to insufficient bond strength with the underlying metal layer.
FIG. 1 is a cross sectional view of a progressing cavity pump/motor 10 described in commonly owned U.S. Patent Publication No. 2009/0098002. This patent publication describes hardfacing the rotor 12 of the progressing cavity pump/motor 10, by roughening the surface of the rotor body prior to flame spraying a metallic coating material onto the roughened surface. FIG. 2 is an elevational view of the rotor 12 shown separately. As shown in FIG. 2, the rotor 12 has the shape of a helix and includes a plurality of crests 20 and valleys 22. The distance 23 between two successive crests or valleys is known as the pitch. The rotor 12 has a major diameter 19, the diameter of the circle circumscribed by the crests 20 upon rotation of the rotor 12. The minor diameter 21 of the rotor 12 is the diameter of the circle circumscribed by the valleys 22 of the rotor.
As shown in FIG. 1, the dimensions of the rotor 12 and stator 14 are coordinated such that the rotor 12 tightly fits into the bore 15 and a number of individual pockets or cavities 13 are formed which are inwardly defined by the rotor 12 and outwardly by the stator 14. Upon rotation of the rotor 12 in the operating direction, the cavities 13 and their contents are pushed spirally about the axis of the stator 14 to the output end of the pump. The seal between the cavities is made possible by an interference fit between the rotor 12 and the elastomeric material of the stator 14. Thus, any surface roughness of the rotor will wear the elastomeric material 14, which requires maintenance of the pump 10.
Flame sprayed hardfacings generally have a grainy surface. Leaving this surface untreated will quickly wear out the stator of the pump. Reducing surface roughness may either be effected by polishing or by fusing of the particles. Polishing involves the use of abrasives. This can be a lengthy and inefficient process with some hardfacing materials. Fusing includes the application of heat to the rough surface at a temperature that melts and fuses the grainy particles in order to make the grainy surface continuous.
The heat treatment may either be applied using flame or induction heating. Due to its helical shape, the main challenge in reducing the surface roughness of the rotor 12 using heat treatment is to evenly distribute the heat throughout the rotor 12 such that the outer surface of the rotor 12 is uniformly heated both at the crests and the valleys, in order to avoid localized overheating or insufficient heating of the rotor, both of which will lead to uneven fusing of the hardcoating.
With flame heating, it is impossible to precisely control the direction of the flame to evenly heat the rotor 12, without leaving unfused areas on the outer surface of the rotor 12, especially since the rotor 12 has a helical shape.
FIG. 3 is an example of induction heating a rotor 12 using a conventional induction coil 24. Conventional induction heating methods include advancing the rotor 12 through the inner circumference of an induction coil 24, as shown in FIG. 3. The induction coil 24 includes a plurality of loops and has a substantially cylindrical interior shape. As the rotor 12 passes through the conventional induction coil 24, a magnetic field is applied from the plurality of waveguide loops onto the rotor 12. The inner diameter of the induction coil is greater than the major diameter 19 of the rotor 12.
However, this method is not efficient, and does not heat the rotor 12 evenly, leading to unfused areas on the rotor. This is due to the spacing between the rotor 12 and the induction coil being lower at the crests 20 than in the valleys 22, which results in the magnetic field being stronger at the crests 20 and more heating of the crests 20 than the valleys 22.
It is, therefore, desirable to provide a system and method for evenly fusing a flame sprayed hardcoating on a helical rotor by induction heating.